In electronic equipment that operates using a battery as a power supply, to ensure stable power, a charging circuit charges the battery with power supplied from an AC adapter or a USB power supply. Further, the battery and charging circuit both function as a power supply. In such electronic equipment, a system load must be stably operated by the charging circuit and battery.
FIG. 5 illustrates a conventional charging circuit. The charging circuit 1 is supplied with power from a power supply 2, which is a power supply device such as an AC adapter or a USB power supply. The charging circuit 1 has an output terminal To coupled to a battery 3 and a system load 4 via a current detection resistor R1. The battery 3 is supplied with charge voltage and charge current from the output terminal To. The system load 4 uses the battery 3 and the charging circuit 1 as a power supply.
The current detection resistance R1 has two terminals coupled to input terminals Ti1 and Ti2 of the charging circuit 1. Accordingly, the input terminal Ti2 is supplied with charge voltage Vbat from the battery 3, and the input terminals Ti1 and Ti2 are supplied with the voltage between the two terminals of the current detection resistor R1.
In the charging circuit 1, the input terminals Ti1 and Ti2 are coupled to two input terminals of a current detection amplifier 5. The current detection amplifier 5 amplifies the potential difference between the two terminals of the current detection resistor R1 and supplies an output voltage, which indicates the amplification result, to a negative input terminal of a constant current control amplifier 6.
The constant current control amplifier 6 has a positive input terminal supplied with a reference voltage Vs. Further, the constant current control amplifier 6 provides a drive circuit 7 with an output signal that is in accordance with the potential difference between the reference voltage Vs and the output voltage of the current detection amplifier 5.
The input terminal Ti2 is further coupled to ground GND via resistors R2 and R3. Voltage at node N10 between the resistors R2 and R3 is supplied to a negative input terminal of a constant voltage control amplifier 8. The constant current control amplifier 6 has a positive input terminal supplied with the reference voltage Vs. The constant voltage control amplifier 8 provides the drive circuit 7 with an output signal that is in accordance with the potential difference between the reference voltage Vs and the voltage at node N10.
The drive circuit 7 performs a constant voltage output operation or a constant current operation based on the output signal of the constant current control amplifier 6 and the output signal of the constant voltage control amplifier 7. The output signal of the drive circuit 7 is provided to the output terminal To.
Based on the operational states of the constant current control amplifier 6 and constant voltage control amplifier 8, an operation detector 9 detects whether or not the drive circuit 7 is performing a constant current output operation or a constant voltage output operation and provides its detection signal to a sequence controller 10.
The sequence controller 10 sequence-controls the operation of each of the constant current control amplifier 6 and the constant voltage control amplifier 8 based on the detection signal of the operation detector 9.
The operation for charging the battery 3 with the above-described charging circuit 1 will now be described with reference to FIGS. 3 and 4. FIG. 3 illustrates the charge voltage and the charge current in relation with the elapsed time for a case in which the time desired for the battery voltage (charge voltage) to reach 4.2 V from a fully discharged state is 870 minutes. FIG. 4 illustrates the relationship between the charge voltage and the charge current for this case.
When the battery 3 is in a discharged state and the charging circuit 1 performs a charging operation, the drive circuit 7 supplies a short circuit battery current Ishort, which is a very small constant current, and charging is performed until the charge voltage Vbat reaches a short circuit battery voltage Vshort of 2 V.
The charging is continued with a preliminary charge current of approximately 100 mA. When the charge voltage Vbat reaches a preliminary charge voltage of a little less than 3 V after about 30 minutes, a charge current Ichg, which is a constant current of 450 mA, is supplied to the battery 3 to perform fast charging.
When the charge voltage Vbat reaches the fully charged voltage Vreg of 4.2 V, the charging circuit 1 starts a constant voltage charging operation to gradually decrease the charge current Ichg to a charge completion current Itaper (fully charged state) of 100 mA.
When the system load 4 is driven and the consumption current of the system load 4 is greater than the output current of the drive circuit 7, the lacking current is supplied from the battery 3 to the system load 4. When the power supply 2 is not coupled to the charging circuit 1, the battery 3 supplies the consumption current of the system load 4.
When the system load 4 is supplied with consumption current from the battery 3 in a state coupled to the power supply 2, the charge voltage Vbat decreases from the fully charged voltage Vref, and the charging circuit 1 shifts to a constant current charging operation that uses a constant charge current Ichg of 450 mA. Then, the charge current Ichg is used to supply the system load 4 with consumption current, and the battery 3 is charged with the surplus current obtained by subtracting the consumption current from the charge current Ichg.
Normally, the charge current Ichg of the charging circuit 1 is preferably set to be greater than the maximum consumption current of the system load 4. This allows the system load 4 to be stably supplied with consumption current from the charging circuit 1 even if the battery 3 is removed from the charging circuit 1 (i.e., system load 4).
However, for example, when product development of a power supply IC including a charging circuit is carried out in parallel with product development of a system device, there may be case in which the charging current Ichg of the charging circuit 1 cannot be set to be greater than the maximum consumption current of the system load 4.
In such a case, when the system load 4 operates in a state in which the battery 3 is removed and the consumption current becomes 100 mA, the charging circuit 1 performs the constant voltage control operation with the output voltage of 4.2 V. As the consumption current of the system load 4 increases and the voltage at the input terminal Ti2 decreases, the charging circuit 1 outputs the charge current Ichg of 450 mA. Subsequently, the charging circuit repeats the constant voltage control state and the constant current control state in accordance with changes in the consumption current.
When the consumption current of the system load 4 exceeds the charge current Ichg of the charging circuit 1, a system abnormality may occur.